Ultrahigh Vacuum Cryostat System for Extended Low

Proceedings of the 12th International Spacecraft Charging and Technology Conference 1

Abstractmdash The range of temperature measurements have been

significantly extended for an existing space environment

simulation test chamber used in the study of electron emission

sample charging and discharge electrostatic discharge and

arcing electron transport and luminescence of spacecraft

materials This was accomplished by incorporating a new two-

stage closed-cycle helium cryostat which has an extended sample

temperature range from lt40 K to gt450 K with long-term

controlled stability of lt05 K The system was designed to

maintain compatibility with an existing ultrahigh vacuum

chamber (base pressure lt10-7 Pa) that can simulate diverse space

environments These existing capabilities include controllable

vacuum and ambient neutral gases conditions (lt10-8 to 10-1 Pa)

electron fluxes (5 eV to 30 keV monoenergetic focused pulsed

sources over 10-4 to 1010 nA-cm-2) ion fluxes (lt01 to 5 keV

monoenergetic sources for inert and reactive gases with pulsing

capabilities) and photon irradiation (numerous continuous and

pulsed monochromated and broad band IRVISUV [05 to 7 eV]

sources) The new sample mount accommodates 1 to 4 samples of

1 cm to 25 cm diameter in a low temperature carousel which

allows rapid sample exchange and controlled exposure of the

individual samples Custom hemispherical grid retarding field

analyzer and Faraday cup detectors custom high speed high

sensitivity electronics and charge neutralization capabilities used

with lt50 pA lt5 micros lt3103 electronspulse pulsed-beam sources

permit high-accuracy electron emission measurements of extreme

insulators with minimal charging effects In situ monitoring of

surface voltage arcing and luminescence (250 nm to 5000 nm)

have recently been added

Index Termsmdashmaterials testing electron emission space

environment low temperature ultrahigh vacuum cryostat

instrumentation

I INTRODUCTION

o better understand the effects of the space environment

on materials used in spacecraft construction it is important

to test these materials under highly controlled laboratory

conditions When simulating the space environment three key

conditions to consider are pressure radiation and temperature

Research was supported by funding from the NASA Goddard Space Flight

Center Justin Dekany Robert Johnson Greg Wilson Amberly Evans and JR

Dennison are with the Materials Physics Group in the Physics Department at

Utah State University in Logan UT 84322 USA (e-mail

jdekanyphyxgmailcom bj68127yahoocom GregdWilsongmailcom

ambevaaggiemailusuedu JRDennisonusuedu ) Dekany Wilson and

Evans are graduate students in the USU Physics Department Dennison is a

professor in that department Johnson is an undergraduate student in the

Mechanical Engineering Department at USU

A space environment simulation chamber operating at

ultrahigh vacuum with electron ion and photon flux

capabilities [1] has been extended to a wider operational

temperature range allowing simulation of a broader range of

space environment temperatures

Electron transport and emission properties are central in

understanding modeling and mitigating spacecraft charging

concerns they are particularly susceptible to temperature

changes often exhibiting Arrhenius behavior ~[(1kBT) exp(-

EkBT)] For example one of the mechanisms for exciting

electrons into the conduction band is through thermal

excitation [2] As the temperature decreases thermally

excited electrons are less prone to move through an insulating

material leading to greater likelihood for charge build up In

the case of a spacecraft which is exposed to varying amounts

of electron flux densities and electron energies this

environment can cause isolated regions made primarily of

insulating material to electrostatically discharge Electricalmdash

and in some cases mechanicalmdashfailure caused by Electrostatic

Discharge (ESD) is the leading source of damage to spacecraft

due to the space environment [3] Understanding these effects

by better simulating the space environment over a broader

range of temperatures encountered in space is the driving force

behind these chamber modifications

II EXPERIMENTAL TEST CHAMBER DESIGN

An existing space environment simulation chamber has been

designed to allow for a wide range of electron flux and energy

bombardment while maintaining ultrahigh vacuum and

continuous control of sample temperature The capabilities of

the chamber are summarized first The focus of this paper is

the design of the low temperature stage and the associate

sample mounting system An example of the use of the new

low temperature stage for electron-induced luminescence and

arcing experiments demonstrates the versitility of the design

A Electron Emission Test Chamber

The electron emission test chamber uses standard

mechanical and turbomolecular pumps (V see legend in Fig

1) for roughing and an ion pump (W) for continuous

maintenance-free operation (base pressure of lt10-7

Pa)

Absolute pressure is monitored with Convectron and ion

gauges (X) Partial pressure is measured with a residual gas

analyzer (Y)

Electron flux is produced with a High Energy Electron

Diffraction (HEED) gun (Kimball Model EGPS-21B)1(A)

which provides a monoenergetic beam with a flux of

Ultrahigh Vacuum Cryostat System for

Extended Low Temperature Space Environment

Testing

Justin Dekany Robert H Johnson Gregory Wilson Amberly Evans and JR Dennison

T


~1pAcm2 to 1 microAcm

2 over an energy range of 500 to

3000plusmn001 keV While the gun can produce a focused pulsed

beam for the experiments described here it was used in a

continuous beam mode with a broad defocused beam of ~2 cm

diameter FWHM The current is monitored in real-time using

an in situ Faraday cup (G) located in the center of the sample

mount (D) Currents were measured using a fast sensitive

picoammeter with lt02 pA resolution [5] Additional lower

energy electron gun sources (5 eV to 5 keV) ion sources (100

eV to 5 keV) and photon sources (~150 nm to 2000 nm) are

also available in the chamber

Light detection is accomplished with several cameras All

detectors have been calibrated in situ using NIST traceable

methods allowing for absolute spectral radiance values to be

obtained The cameras were positioned with views through

vacuum port windows to get a clear view of the sample this

allowed for data collection of photon emission from the

sample as a result of the cathodoluminessence and arcing

caused by the electon beam interacting with the sample [4]

A Single Lens Reflex (SLR) CCD camera (O) (Cannon

EOS Rebel XT DS126071) took visible light data ranging

from ~400 nm to 700 nm with 10 Mpixel images The camera

typically operates at 30 s shutter speeds at full aperture with a

55 mm lens giving it an average spectral response of ~4109

Counts(Wcm2srm) A CCD video camera (L) (Xybion

ISG-780-U-3) was sensitive to light from ~400 nm to 900 nm

and collected data at 30 frames per sec The spectral response

for this detector is ~41010

Counts(Wcm2srm) using a 55

mm lens An InGaAs video camera (M) was sensitive 800 nm

to 1700 nm wavelengths with maximum spectral response for

longer wavelengths This InGaAs video camera was operated

at ambient room temperature collecting data at 601 frames per

second It has a spectral response of ~1109

Counts(Wcm2srm) using a 35 mm lens An InSb video

camera (N) acted as a low spatial resolution IR detector

(320x280 pixels) The photon response of this detector

increases in sensitivity with increasing wavelength ranging

from 1 m to 55 m This camera operates at liquid nitrogen

temperatures collecting data at varying integration times

ranging from ~10 Hz to ~30 Hz depending on the band pass

filters used This detector has an spectral responsivity of

~7107 Counts(Wcm

2srm)

(a)

(b)

(c)

Fig 1 Experimental test chamber (a) Chamber exterior view with cutaway showing the various analysis component lines of sight The electron gun is

positioned on axis with the sample plane (b) Cryostat chamber mount showing the electron beam trajectory (red) (c) Cutaway view of cryostat showing the

first stage radiation shroud and second low temperature stage with sample mount and pedestal (red)


Two fiber optics-based spectrometers (K) provide

UltravioletVisibleNear Infrared photon spectral

measurements from ~250 nm to 1700 nm The UVVis

spectrometer (Stellarnet 13LK-C-SR ~200 nm to 1080 nm)

has a wavelength resolution of ~1 nm while the NIR

spectrometer (Stellarnet RW-InGaAs-512 ~1000 nm to 1700

nm) has a ~3 nm resolution Both detectors are housed in a

piezoelectric cooler which maintains -20 K from ambient A 4

cm diameter MgF collection optic gathers the light emitted by

the sample and focuses the signal to a 1 m fiber optic cable

routed to the spectrometers

Although not used for the measurements described in this

paper the new cryostatsample mount is compatible with

electron detection capabilities as well The primary detector

for emission studies is a custom hemispherical grid retarding

field analyzer (HGRFA) with a retarding-field analyzer grid

system for emitted-electron energy discrimination between

backscattered electrons (energies gt50 eV) and secondary

electrons (energies lt50 eV) By ramping the grid bias energy

spectra of the emitted electrons can also be measured using

this detector The HGRFA features a fully-encasing

hemispherical collector for full capture of emitted electrons

that is particularly well suited and calibrated for absolute yield

measurements The HGRFA can be positioned in front of a

single sample mounted at the end of the cryostat

B Low Temperature Stage Design

A closed-cycle helium cryostat has been added to the space

environment simulation chamber to extend the operational

temperatures for sample testing from lt40 K to gt450 K

Temperature is maintained to plusmn05 K using a standard

computer-controlled PID temperature controller (RMC

Cryosystems CR31-21) under Labview command and

platinum RTDs (I) mounted on the sample holder low

temperature stage and cryostat radiation shield A large mass

radiation shield (J) was attached to the cryostatrsquos first stage

(thermal load capacity of ~10 W at ~80 K) and a sample

mounting stage was attached to the cryostats second stage

(thermal load capacity of ~1 W at ~20 K) these large thermal

masses helped maintain a constant sample temperature Due

to radiative heat transfer from the chamber walls the addition

of a multilayer thermal insulation blanket (R) wrapped around

the first stage radiation shield (J) was required This blanket

consists of five sheets of thin conducting material separated by

a thin mesh of insulating material With this addition to the

apparatus the sample holder which is mounted to the cryostats

second stage sample pedestal (B) can then reach 40 K a

temperature comparable to passively cooled spacecraft in

standard orbit Direct measurement of a sample confirmed a

lt2 K gradient between the samples and sample holder on

which the temperature probe was mounted Once the chamber

was down to pressures of lt510-3

Pa the cryostat cooled the

sample at a rate of ~ 1 Kmin reaching the lowest temperature

of ~40 K in about 4 hr this temperature can be sustained for

weeks Heating the sample is accomplished using a

combination of control and bulk heaters attached to the

radiation shroud and sample mount Once activated the

temperature controller can heat the sample at a rate of ~ 15

Kmin and can maintain any intermediate temperature from

lt40 to gt 450 K A typical cooling and heating profile is

shown in Fig 7

Fig 2 Cryostat connected to the electron emission test chamber

Fig 3 View of cryostats second stage sample pedestal and interchangeable

sample holder which attaches to the cryostat Shown is a sample holder for

four 1 cm diameter samples and a central Faraday cup and for a single 25 cm

diameter sample

Fig 4 Interchangeable sample holder with four 1 cm samples attached to the

sample mount


The cryostat (see Fig 1(b)) can be removed from the

electron emission chamber and installed on other vacuum

chambers that have an available 10 cm or 15 cm port

C Sample Carousel Design

The sample mount (D) has a versatile design that allows for

a variety of configurations and sample sizes Multiple sample

holders (F) can be quickly interchanged with the use of spring-

loaded electrically-isolated electrode connectors (H) as

shown in Fig 3 The sample stage is electrically isolated with

a Kapton spacer and PEEK screws but maintains good

thermal contact with the sample stage The sample stage has a

large wiring cavity (D) to facilitate various low-noise

electrical connections in addition to allowing room for bulk

and control heaters

In one configuration four 1 cm diameter samples can be

installed with an optional in situ Faraday cup (G) in the center

location allowing for real-time monitoring of electron beam

current (see Fig 4) Samples in this configuration are

mounted on (100 plusmn 01) mm diameter Cu cylinders usually

using UHV compatible low-temperature conductive epoxy

(Masterbond EP21TDCS-LO) The Cu cylinders are mounted

in sample blocks using ceramic pins or 100 microm diameter

sapphire spheres held in place with set screws to provide

electrical isolation (see Fig 4) Electrical connections to the

sample are made via one or more spring loaded pins from the

rear allowing the sample current(s) to be monitored Using

this configuration a sample mask selection gear (E) coupled

to an external rotation translation feedthrough (U) allows

masking of the samples not being tested (see Fig 5) this

minimizes potential sample charging of these samples The

sample masks also minimizes the amount of sample area

exposed to the higher temperature chamber walls since the

mask is attached to the 80 K second stage of the cryostat

Larger samples of up to 25 cm in diameter or 25 cm by 25

cm square can be tested using a different sample mount (D) as

shown in Fig 6

III APPLICATIONS

To highlight the capabilities of the low temperature cryostat

incorporated into the space environment simulation test

chamber we present data from a study of the temperature

dependence of electron-induced luminescence and arcing from

spacecraft materials With the various imaging detectors all

focused at the sample (see Fig 1(a)) a wide range of spectral

analysis is possible [4] Operation in a closed chamber in a

dark room makes it possible to measure very low intensity

sources

An example of this is shown in still frames captured with a

CCD video camera Figure 8(a) shows a 1 cm sample which

is illuminated using a dim fiber optic timing light to show the

region to be exposed to the electron flux Figures 8(b) and

8(c) show a comparison of the effect temperature has on this

material the glow at 293 K is hardly detected with this

imaging camera but as the temperature is decreased to 40 K

intensity increases and a prominent glow is clearly evident If

enough charge is built up in the material arc events were

observed [6] where the charge in the sample found a

conduction path to the edge of the grounded sample holder

(F) The very intense optical signature of the arc has been

captured in a video frame shown in Fig 8 (d) Figure 9 shows

simultaneous measurements from an arc event as recorded by

the electrometer and the CCD video camera Here a constant

current density of 1 nAcm2 was incident on the 1 cm sample

and at ~91 s elapsed time the sample current spiked to over 40

nA Additional measurements from a digital storage

oscilloscope showed that these arcs were typically less than 1

micros in duration The image from the CCD video camera

Fig 5 Sample selection gear controlled using an external rotation translation

(U) The gearsample mask to access the Faraday cup (G) and one of four 1

cm diameter samples is shown

Fig 6 Face plate with a 25 cm diameter sample mounted Note radiation

blanket wrapped around first stage radiation shield (J) of cryostat

Fig 7 Typical sample stage cooling and sample heating curves with multiple

sustained temperatures shown during heating cycle


provides optical confirmation of the electrical signature and

provides additional information about the spatial location of

the arc Note the currents from the central region of the

sample and from the sample edge region are measured

independently as shown in Fig 9(b)

ACKNOWLEDGEMENT

We gratefully acknowledge contributions to instrumentation

from Tamara Jeppsen of the Materials Physics Group Michael

Taylor for the use of infrared and CCD video cameras and

useful discussions with Robert Meloy and Charles Bowers of

NASA Goddard Space Flight Center This work was

supported by a project for the James Webb Space Telescope

Electrical Systems Group through the NASA Goddard Space

Flight Center

REFERENCES

[1] WY Chang JR Dennison Jason Kite and RE Davies ldquoEffects of

Evolving Surface Contamination on Spacecraft Chargingrdquo Paper AIAA-

2000-0868 Proceedings of the 38th American Institute of Aeronautics

and Astronomics Meeting on Aerospace Sciences (Reno NV 2000)

[2] JR Dennison ldquoThe Dynamic Interplay Between Spacecraft Charging

Space Environment Interactions and Evolving Materialsrdquo Proc of the

12th Spacecraft Charging Techn Conf (Kitakyushu Japan May 14-18

2012)

[3] RD Leach and MB Alexander ldquoFailures and anomalies attributed to

spacecraft chargingrdquo NASA Reference Publication 1375 NASA

Marshall Space Flight Center August 1995

[4] Amberly Evans Gregory Wilson Justin Dekany Alec M Sim and JR

Dennison ldquoLow Temperature Cathodoluminescence of Space

Observatory Materialsrdquo Proc of the 12th Spacecraft Charging Techn

Conf (Kitakyushu Japan May 14-18 2012)

[5] CD Thomson V Zavyalov and JR Dennison ldquoInstrumentation for

Studies of Electron Emission and Charging from Insulatorsrdquo

Proceedings of the 8th Spacecraft Charging Techn Conf (NASA

Marshall Space Flight Center Huntsville Al October 2003)

[6] G Wilson JR Dennison A Evans and J Dekany ldquoElectron Energy

Dependent Charging Effects of Multilayered Dielectric Materialsrdquo Proc

of the 12th Spacecraft Charging Techn Conf (Kitakyushu Japan May

14-18 2012)

Robert Johnson is currently a student in Mechanical

Engineering at Utah State University Logan He has

worked with the Materials Physics Group for a year on

thermal vacuum chamber design related to spacecraft

charging

Justin Dekany is currently a graduate student at Utah

State University in Logan UT pursing an MS in

physics He received a BS degree in physics from USU

in 2010 He has worked with the Materials Physics

Group for four years on electron transport

measurements electrostatic discharge tests electron

emission measurements and luminescence studies

related to spacecraft charging He has been the Lab

Manager for the Materials Physics Group for the last

two years

Fig 8 CCD video camera images (a) Timing light frame showing the sample location to be observed (b) 293 K data with electron beam on sample (c) 40 K

data with electron beam on sample (d) 40 K data with electron beam on sample showing an arc event

(a) (c) (d) (b)

Fig 9 Measured signatures of arcing (a) Electrometer current data (b)

CCD VisNIR video camera integrated sample intensity data showing the

increase in spectral radiance due to the arc event (surface glow data shown in

green edge glow data shown in red)


J R Dennison received the BS degree in physics from

Appalachian State University Boone NC in 1980 and

the MS and PhD degrees in physics from Virginia

Polytechnic Institute and State University (Virginia

Tech) Blacksburg in 1983 and 1985 respectively He

was a Research Associate with the University of

MissourimdashColumbia before moving to Utah State

University (USU) Logan in 1988 He is currently a

Professor of physics at USU where he leads the

Materials Physics Group He has worked in the area of

electron scattering for his entire career and has focused

on the electron emission and resistivity of materials

related to spacecraft charging for the last two decades

Amberly Evans is currently a graduate student at Utah

State University in Logan UT pursing an MS in physics

She received BS degrees in physics and chemistry from

USU in 2012 She has worked with the Materials

Physics Group for five years on electron emission

luminescence and resistivity studies and on MISSE

retrieval and post-flight analysis of SUSpECS Much of

her work has focused on optical scattering of spacecraft

materials

Greg Wilson is currently a graduate student at Utah


physics He received BS degrees in physics and

mathematics from USU in 2011 He has worked with

the Materials Physics Group for two years on electron

emission and luminescence studies related to spacecraft

charging He also developed a composite model for

electron range over a wide range of incident energies

applicable to diverse materials


~1pAcm2 to 1 microAcm

2 over an energy range of 500 to

3000plusmn001 keV While the gun can produce a focused pulsed

beam for the experiments described here it was used in a

continuous beam mode with a broad defocused beam of ~2 cm

diameter FWHM The current is monitored in real-time using

an in situ Faraday cup (G) located in the center of the sample

mount (D) Currents were measured using a fast sensitive

picoammeter with lt02 pA resolution [5] Additional lower

energy electron gun sources (5 eV to 5 keV) ion sources (100

eV to 5 keV) and photon sources (~150 nm to 2000 nm) are

also available in the chamber

Light detection is accomplished with several cameras All

detectors have been calibrated in situ using NIST traceable

methods allowing for absolute spectral radiance values to be

obtained The cameras were positioned with views through

vacuum port windows to get a clear view of the sample this

allowed for data collection of photon emission from the

sample as a result of the cathodoluminessence and arcing

caused by the electon beam interacting with the sample [4]

A Single Lens Reflex (SLR) CCD camera (O) (Cannon

EOS Rebel XT DS126071) took visible light data ranging

from ~400 nm to 700 nm with 10 Mpixel images The camera

typically operates at 30 s shutter speeds at full aperture with a

55 mm lens giving it an average spectral response of ~4109

Counts(Wcm2srm) A CCD video camera (L) (Xybion

ISG-780-U-3) was sensitive to light from ~400 nm to 900 nm

and collected data at 30 frames per sec The spectral response

for this detector is ~41010

Counts(Wcm2srm) using a 55

mm lens An InGaAs video camera (M) was sensitive 800 nm

to 1700 nm wavelengths with maximum spectral response for

longer wavelengths This InGaAs video camera was operated

at ambient room temperature collecting data at 601 frames per

second It has a spectral response of ~1109

Counts(Wcm2srm) using a 35 mm lens An InSb video

camera (N) acted as a low spatial resolution IR detector

(320x280 pixels) The photon response of this detector

increases in sensitivity with increasing wavelength ranging

from 1 m to 55 m This camera operates at liquid nitrogen

temperatures collecting data at varying integration times

ranging from ~10 Hz to ~30 Hz depending on the band pass

filters used This detector has an spectral responsivity of

~7107 Counts(Wcm

2srm)

(a)

(b)

(c)

Fig 1 Experimental test chamber (a) Chamber exterior view with cutaway showing the various analysis component lines of sight The electron gun is

positioned on axis with the sample plane (b) Cryostat chamber mount showing the electron beam trajectory (red) (c) Cutaway view of cryostat showing the

first stage radiation shroud and second low temperature stage with sample mount and pedestal (red)






























































shown in Fig 7





diameter sample


sample mount















and control heaters






















shown in Fig 6

III APPLICATIONS









sources


































ACKNOWLEDGEMENT








Flight Center

REFERENCES








2012)















14-18 2012)





charging










two years



(a) (c) (d) (b)



























materials







































































shown in Fig 7





diameter sample


sample mount















and control heaters






















shown in Fig 6

III APPLICATIONS









sources


































ACKNOWLEDGEMENT








Flight Center

REFERENCES








2012)















14-18 2012)





charging










two years



(a) (c) (d) (b)



























materials
























and control heaters






















shown in Fig 6

III APPLICATIONS









sources


































ACKNOWLEDGEMENT








Flight Center

REFERENCES








2012)















14-18 2012)





charging










two years



(a) (c) (d) (b)



























materials
















ACKNOWLEDGEMENT








Flight Center

REFERENCES








2012)















14-18 2012)





charging










two years



(a) (c) (d) (b)



























materials
































materials










Documents

Ultrahigh Vacuum Cryostat System for Extended Low